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  1. #1
    Join Date
    Aug 2006
    Posts
    10
    Just bought a new home.
    I am unhappy with the A/C system.
    The A/C contactor that installed it is of no help.

    I would like two things:
    1] If it is not against the forums rules...Recommendations
    For a honest Local Tech That KNOWS what he is doing in
    the Jacksonville Fl area to do a Manual J as well as to
    Checkout the systems performance & wiring

    2] Confirmation that I an not crazy...
    The home is 3200Sq, two story. Uptairs is a 3.5 ton Lennoz 12 seer zonned to the upstairs & downstairs bedrooms. The rest of the house downstairs is on a 2.5 ton lennox 12 seer.

    Outside temp all this week has been a high of 92-94, low of 84-86. The downstairs unit runs from 9 am to 11 pm nonstop and can NOT maintain 77. In fact it will go as high as 81.
    The humidity sensor reads high of 55 lo of 46. If I run it alone or along with the upstairs sytem this does not change.

    It is not so much that it is costing me ALOT of $ to run this but that we are uncomfortable and doing so.

    I have some basic A/C skills and as such am sure that there are no leaks in the duct work (but not if they are the right size), filters are new, evap & cond coils clean and unobstructed.

    Also I have hard specs on the system:
    Low side PSI = 69 Line temp* = 64 Chart Temp = 40.5
    High side PSI= 265 Line temp* = 93 Chart Temp = 121
    *Temp was optained using Infrared Thermometer
    Outside temp was 90
    Inside temp was 78
    Temp differential at airhandler 20.5

    I welcome anyone who can offer suggestions to do so.
    thank you.

  2. #2
    Join Date
    Aug 2006
    Posts
    1
    Guessing R22 non/txv since it's obvious that the superheat is not being held constant.

    The superheat as described is awfully high. The low-side pressure of 69 PSI indicates the freon is boiling at about 40 in the compressor, and the low side temperature of 64 was directly measured. The superheat, 64 - 40 = 24 is on the high side for the conditions. With the indoor humidity around 50%, the wet-bulb would drop from 77 down to around 64 degrees. Giving a target superheat of 9. So with the actual superheat (24) being considerably higher than the target (9), the system appears to be undercharged.

    Assume the house is around 80 inside and you have a tech adding refridgerant. The low side pressure will rise to a maximum, maybe 80 PSI, but then fall at the indoors becomes cooler. It will finally stabilize around 70 PSIG assuming the house is cooled to around 70 degrees.

    70 PSIG with house at 80, not normal.
    70 PSIG with house at 70, pretty normal.

    Bottom line.. system appears to be undercharged. Call the guy's boss and tell him to send somebody with a jug of refrigerant who understands the importance of taking indoor wet bulb, low side line, compressor boiling, and outdoor ambient temperatures. Otherwise he's guessing.

    If yours is a new installation, then getting the freon charge wrong is really dumb since it can simply be weighed into a new system.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,898
    By the readings you gave, under chage is not the problem.

    You posted a 28 subcool, way too much.

    Those laser therms aren't real acturate on copper lines.

    You didn't post how big the rest of the downstairs is, it could be indersized, and the bedroom unit over sized.

    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    Based on square feet only it sounds like you have enough air conditioner to do the job. First, beenthere is correct; infrared thermometers are worthless on shiny surfaces and only ballpark on anything else. Next, new homes tend to have high humidity from construction moisture being present. Glues, concrete, etc. need time to dry and some of your system capacity will be used for removing latent heat (humidity) instead of sensible heat (temperature) until everything dries out. Give it at least a couple weeks and see if there is any improvement. If not, call the contractor back.

    If your system runs continuously when the outside temp is 90+ this is a good thing. This should remove maximum moisture from the house and make it more comfortable. I personally think that it is much more comfortable at 80 degress and 38% Rh than at 72 degrees and 58% Rh.


  5. #5
    Join Date
    Aug 2006
    Posts
    10
    Thanks fir all the help.
    More info for U.


    The house is 3200 under air, downstairts ac services about 2200 Sq ft.

    The house has now been occuppied for 7 months.

    The sytem is an Expansion Valve.

    I do have Digital stick Thermometers but none gave me reaings on the liquid line. I do not think that I could get a good contact. What should I use?

    According to Lennox the psi should read 79/224 @ 90 outside. A you can see I have a lower low side & A higher High side. Why?

    FYI: They sent out another tech since these posts.
    He recovvered freon so that the gauges now read:
    65/190 @80 Deg. Lennox paper says @ 80 it should read 77/180
    +/-5 low & +/- 10 high. He did so slowly over the course of 2 hrs removing only 5 PSI hifh side at a time, stabilizing the system & then reading the temp diff at the air handler.
    He said he was looking for the lowest Temp point? Anyway he started with 21 deg diff & through the whole process it only changed to 22 deg diff.
    Is this normal?
    That is in my mind a HUGE difference in system
    charge for very little perfomance change.



    [Edited by selkov on 08-10-2006 at 11:05 AM]

  6. #6
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    Those pressures in the Lennox literature are based on average conditions. They do not take into account the dewpoint in your home and individual system characteristics. It sounds like the tech knew what he was doing. If you are getting 22 Deg across the system it should be about perfect.

  7. #7
    Join Date
    Aug 2006
    Posts
    10
    How do we take into account the dew pointint in my home?

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,898
    Originally posted by selkov
    Thanks fir all the help.
    More info for U.


    The house is 3200 under air, downstairts ac services about 2200 Sq ft.




    [Edited by selkov on 08-10-2006 at 11:05 AM]
    So the 3.5 ton is for 1300 sq ft, and the 2.5 ton is for 2200 sq ft.

    Sounds like they accidentally switched the units when they installed them.

    You might want to remind youor builder about what size unit is conditioning which parts of teh house.
    Contractor locator map

    How-to-apply-for-Professional

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  9. #9
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    What I was trying to say is that you can't just look at the pressure chart in the booklet and charge to those pressures. Those pressures are just a guideline. I don't even know why they are printed there. What you charge to is called Subcooling. It has to do with comparing the liquid line temperature with the saturated temp of the refrigerant in the condenser, thus ensuring the proper amount of liquid refrigerant in the condenser coil during certain outside air conditions. The pressures will vary somewhat according to the cooling load inside your house. The cooling load is a function of BOTH sensible (temperature) load and latent (humidity) load. The combination of sensible and latent heat can be measured using the dewpoint. Dewpoint is affected by factors such as house construction and indoor fan speed. A slower fan speed will decrease the latent energy level (humidity). A faster fan speed decreases the sensible energy level (temp) more than the latent. An average dewpoint in a house is around 50-55 degrees. The pressure chart assumes this, but is that what your home actually reads at the time of checking?

    You can see that there is a lot more to it than just adding refrigerant until the pressures match a chart.

  10. #10
    Join Date
    Aug 2006
    Posts
    10
    man from trane...thanks.
    I have read some on subcool & superheat.
    This make more sense to me than psi charts.

    But I do not understand how to arrive at a "Target" when
    charging via subcool. One A/C tech said I could not use
    that method because there was no subcool/superheat
    information on the service panel.

    Lennox uses an "Aproach Method" in thier manual. Basically taking ambient temp - LLT and looking for deg diff. Is this OK?

  11. #11
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    I have a Lennox in my home and have read the manual. For my unit, the ACC10, the approach method is for a Thermostatic expansion valve. The subcooling method is for the orifice type expansion valve. I don't know which you have. I had never heard of the approach method before I read this booklet, but if Lennox says to do it, it must be OK.

    The "Target" in subcooling is reached by converting the pressure of the high side refrigerant to a temperature by means of a Temperature/Pressure chart and subtracting the liquid LINE temp (which should be lower) from it. You have to know the pressure in the high side first and so it requires a manifold set to do it. If you don't have a refrigerant certification I would not recommend hooking up gauges to the unit because your neighbors could turn you in to the EPA. And as I mentioned before, it sounds like your unit is properly charged.

    Beenthere has a good point. Are the systems supplying the correct parts of the house? Hard to believe anyone would make that mistake, but I've seen stranger things.

  12. #12
    Join Date
    Aug 2006
    Posts
    10
    Man from trane.....

    I am investigating weather or not the systems are supplying air to the right locations. It was on my list of things to do.


    Perhaps I can clarify what I ment when asking for the "Subcool Target".


    My system has an Expansion valve.
    I understand how to achieve the subcool the actual reading "temperature by means of a Temperature/Pressure chart and subtracting the liquid LINE temp". But for instance if the differance is 15 is that good or bad?
    Does my system require 10 or 12 or 14?
    Where or how do I figure that out?

  13. #13
    Join Date
    Jun 2006
    Location
    Omaha, Nebraska
    Posts
    861
    You say your system has an Expansion valve. I assume you mean a thermostatic expansion valve (TXV) as opposed to a piston or orifice type expansion device. I thought Lennox recommended using the Approach method for a TXV?

    There should be a chart in your installation guide that tells what the subcooling should be, based on outside air temp. In my install guide I think it's 15 degrees subcooling if the OAT is 70-80 degrees. 15 degrees gets you pretty close under almost all conditions. Of course this assumes 70-80 degrees inside the home also.

    [Edited by man from trane on 08-14-2006 at 07:05 PM]

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